The properties of nematic or nematic-cholesteric liquid-crystalline materials whereby they significantly vary their optical properties, such as light absorption, light scattering, birefringence, reflectivity or color, under the influence of electric fields, are widely utilized for electro-optical display elements. The functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the industrial application of these effects in electronic components, liquid-crystalline dielectrics are required which must meet a large number of demands. Chemical resistance to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet regions, and continuous and alternating electric fields, is of particular importance. Industrially usable liquid-crystalline dielectrics are also required to have a liquid-crystalline mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably from 0.degree. C. to 60.degree. C., and the lowest possible viscosity at room temperature, which preferably should not exceed 70.times.10.sup.-3 Pa.s. Finally, they must not have any characteristic absorption in the region of visible light, i.e., they must be colorless.
A number of liquid-crystalline compounds have already been disclosed, which fulfill the stability demands made on dielectrics for use in electronic components, and which are also colorless. These include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'-disubstituted phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684. In both classes of compounds, and also in other known series of compounds with a liquid-crystalline mesophase, there are no individual compounds which form a liquid-crystalline nematic mesophase in the required temperature range of 10.degree. C. to 60.degree. C. Therefore, mixtures of two or more compounds are generally prepared in order to obtain substances which can be used as liquid-crystalline dielectrics. For this purpose, at least one compound having a low melting point and clear point is usually mixed with another compound having a markedly higher melting point and clear point. This normally gives a mixture, the melting point of which is below that of the lower-melting component, while the clear point is between the clear points of the components. Nevertheless, difficulties arise again and again in the preparation of optimum dielectrics, because the components having the high melting points and clear points frequently also impart a high viscosity to the mixtures. As a result, the switching times of the electrooptical display elements produced with these mixtures are extended in a undesirable manner. Moreover, problems are frequently caused by the fact that the mutual solubility of the various components, in particular at room temperature or lower temperatures, is only very limited.